1998 GMC Jimmy Rough idle at start

What would cause a rough idle after sitting for a day or 2? After sitting, say over the weekend, it starts hard, after about 5 minutes or so of cranking it starts. Running very rough and the check engine light comes on.
After running for a while, 5 to 10, minutes it starts to smooth out then completely smooths out and runs great.
I drive about 175 miles a day with my Jimmy. Normally this only happens when sitting for a length of tiime, only when it gets real bad will the hard starting happen over night. The guy I have working on my truck said at one time it was injectors. I started using GM injector cleaner and the problem seems to go away. Then in a week or so I have to dump in another bottle. I've used the same gas from the same station for years and never had a problem before.
Thanks for your help.